Because Tom's argument works both ways. In Islam at least, a major reason for Muhammad's original stance in favour of polygamy was in fact a kind of charity: Muhammad himself married a number of women whose first husband had died in battle (or some other way) and who were left without much to support themselves. It's not something we commonly think of when discussing polygamy, as people always assume the men are just horny bastards who want several hot women instead of just one, but back in the seventh century it was a big factor.